Fix slowness, crashing, error messages and other problems

In English

• Add-on signing in Firefox for Android
• Firefox for Android displays blank web pages
• Some videos won't play in background
• Firefox Health Report - understand your Android browser performance
• I can't play videos or audio on Firefox for Android 5.0 (Lollipop)
• Firefox Marketplace Apps Stop Working on Firefox 47+ for Android
• Avoid and report Mozilla tech support scams